compaction: integrate range keys

[jbowens]

Range keys (#1339) need to be read and written during compactions. Like range deletions, range keys exist separately from point keys and are surfaced through the keyspan.FragmentIterator interface. Here's a sketch of the anticipated integration.

1. When constructing a compaction input iterator, a keyspan.LevelIter is created for each input level that contains at least 1 file with range keys.
2. The individual levels' range key keyspan.LevelIters are wrapped in a keyspan.MergingIter. The MergingIter is configured with a special keyspan.Transform. The Transform is configured with the slice of open snapshots. It partitions a Span's Keys into snapshot stripes, calling rangekey.Coalesce on the Keys of each stripe. We may want to refactor rangekey.Coalesce's interface to only take an input []Keys slice and output *[]Keys pointer. The rangekey.Coalesce calls performs shadowing, dropping unnecessary keys. This Transform is also likely where we should examine in-use key ranges (eg, elideRangeTombstone) to determine whether or not to drop RangeKeyUnset and RangeKeyDelete keys that exist in the last snapshot stripe.
3. The MergingIter is then wrapped in a keyspan.DefragmentingIter configured with the DefragmentInternal defragmenting method. The defragmenting iterator will ensure the fragments persisted to disk are only as fine-grained as required.
4. Finally, the keyspan.DefragmentingIter is wrapped with a keyspan.InterleavingIter to interleave range keys into the main compaction loop. This is similar to what was done for range deletions in compaction: use InterleavingIter to interleave rangedels #1659.
5. When the main compaction loop encounters a RANGEKEYSET, RANGEKEYUNSET or RANGEKEYDELETE, it retrieves the span from the interleaving iterator and adds it to a keyspan.Fragmenter (separate from c.rangeDelFrag). All the truncation logic mirrors range deletions. Range keys should be added to the output sstable using Writer.AddRangeKey (not the Writer.RangeKey{Set,Unset,Delete} methods). Future work may consider removing the keyspan.Fragmenter, but I think it should be deferred for now, and we should track as close to possible to the current handling of range deletions.

[jbowens]

A note that came up in cockroachdb/cockroach#82041 — Compactions and flushes involving inputs containing range keys should calculate a separate inUseKeyRanges using the range-key bounds of sstables. That will ensure we're able to drop ineffective RANGEKEYDELs much sooner.